Variable assign

Hi everyone

I have this code:

<xsl:template match="area[@type='list']">
   <xsl:variable name="file">
      <xsl:choose>
         <xsl:when test="@dinamicxml='yes'">
            <xsl:value-of select="concat($SHARE,'/DPLX/', @descriptor)"/>
         </xsl:when>
         <xsl:otherwise>
            <xsl:value-of select="concat($SHARE, $bizcase/@lang,
'/descriptors/', @descriptor)"/>
         </xsl:otherwise>
      </xsl:choose>
   </xsl:variable>
   <xsl:variable name="descriptor" select="document($file)/descriptor"/>
   <xsl:variable name="multi">
      <xsl:value-of select="$descriptor/@multi"/>
   </xsl:variable>

That doesn't work for the assign of the variable descriptor, it keeps telling
me that it can't find the specified object.
I already checked the value of the variable file, and its correct (it's a path
for a XML file)

When I change the assign of the file variable and the code looks llike this:

  <xsl:variable name="file" select="concat($SHARE, $bizcase/@lang,
'/descriptors/', @descriptor)"/>
  <xsl:variable name="descriptor" select="document($file)/descriptor"/>
  <xsl:variable name="multi">
    <xsl:value-of select="$descriptor/@multi"/>
  </xsl:variable>

Everything works fine.
Why is it that the assign of the node for the variable descritor works well
for this option and for the other it doesn't?
What is diference of assigning the file variable using a choose and not
using?? :-(

Best Regards

JC